Hot pressing of silicon nitride using magnesium silicide

ABSTRACT

A dense polycrystalline silicon nitride body is produced by hot-pressing a particulate mixture of silicon nitride and a magnesium silicide additive.

The invention herein described was made in the course of or under acontract or subcontract thereunder (or grant) with the Department of theAir Force.

This is a division of application Ser. No. 756,083, filed Jan. 3, 1977.

Silicon nitride is a choice candidate material for turbine applicationsbecause of its good high temperature strength and creep resistance, lowthermal expansion coefficient and excellent oxidation resistance. Sofar, the conventional method of producing large specimens of densesilicon nitride is by hot-pressing with the help of an oxide flux, attemperatures greater than 1700° C. Various oxide fluxes or densificationaids, such as MgO, Y₂ O₃, ZrO₂ and Ce₂ O₃, permit the attainment of fulldensity in hot-pressed Si₃ N₄. However, experience has shown that theseoxide additions produce a silicate glass at grain boundaries which has adeleterious effect on the high temperature creep and strength propertiesdue to the softening or melting of the glassy phase at temperaturesranging from about 1000° C. to 1200° C. depending on the oxide fluxadded. Consequently, most efforts to improve the high temperatureproperties of Si₃ N₄ containing an oxide additive(s) have been directedtowards improving the refractoriness of the silicate "glassy" phase bycomposition control and crystallization methods.

In accordance with the present process no oxide additive is used. Also,at the grain boundaries of the present product there appears to be nodetectable glassy phase.

The present invention is directed to hot-pressing a homogeneousparticulate dispersion of silicon nitride and magnesium silicide toproduce a novel dense polycrystalline body of silicon nitride whichsubstantially retains its room temperature mechanical properties atelevated temperatures ranging up to about 1350° C. or higher in air,depending on the purity of the starting powder.

Those skilled in the art will gain a further and better understanding ofthe present invention from the detailed description set forth below,considered in conjunction with the FIGURE, accompanying and forming apart of the specification, which is a graph showing relative densitiesof silicon nitride hot-pressed with magnesium silicide additions at1750° C. for 20 minutes under a pressure of 8000 psi. The amount of Mg₂Si is based on the amount of silicon nitride. The graph illustrates thepresent invention and shows the highly dense bodies of silicon nitridewhich can be produced particularly with additions of about 2% by weightof Mg₂ Si.

Briefly stated, the process of the present invention comprises providingat least a significantly homogeneous particulate dispersion or mixturehaving an average particle size which is submicron of silicon nitrideand magnesium silicide in an amount of about 0.5% by weight to about3.0% by weight based on the amount of said silicon nitride, andhot-pressing said particulate dispersion in an atmosphere of nitrogen ata temperature ranging from about 1600° C. to about 1850° C. under aminimum pressure of about 2000 psi to produce a pressed body having adensity of at least 80% of the theoretical density for silicon nitride.

The silicon nitride powder used in the present process may be amorphousor of the α-type or mixtures thereof. These powders can also containβ-silicon nitride usually in an amount up to about 20 weight % of thetotal amount of silicon nitride.

At present commercially available silicon nitride powder in anysignificant amount is formed by nitridation of silicon powder with theaid of catalysts which always leave CaO, Fe₂ O₃, and Al₂ O₃ asimpurities, in a significant amount, typically about 1 to 2%. Such asilicon nitride powder is not useful in the present process because whenit is hot-pressed, even without an oxide flux, these impurities combinewith SiO₂, which is inherently initially present in silicon nitride orforms on firing, to produce a low melting intergranular glassy phase.

In contrast, the present starting silicon nitride powder issubstantially pure but it can range somewhat in purity. The necessarypurity of the powder used depends largely on the temperatures and loadsat which the final hot-pressed product will be used with the highesttemperatures of use generally requiring the most pure powders.Specifically, with increasingly pure powder the resulting hot-pressedproduct increasingly retains its room temperature properties at hightemperatures, i.e. the more stable are the mechanical properties of thehot-pressed product with increasing temperature.

The present silicon nitride powder may contain certain metallic andnon-metallic impurities in a limited amount and these impurities arebased on the total composition of the starting silicon nitride powder.Specifically, the powder should be free or substantially free ofmetallic impurities which react with SiO₂ or Si and O₂ to form lowmelting intergranular silicate glassy phase in a significant amount.Those impurities which form such a glassy phase include calcium, ironand aluminum and should not be present in a total amount greater thanabout 0.1% by weight. Also, the present silicon nitride powder may havean oxygen content ranging up to about 3% by weight. Normally, the oxygenis present in the form of silica. The amount of excess elemental siliconin the powder should not be present in an amount higher than about 4% byweight because appreciable amounts of residual elemental silicon may beretained in the product, depending on the extent of nitridationoccurring during hot-pressing. Also, any elemental silicon presentshould be of submicron size and should be substantially homogeneouslydispersed throughout the powder. Non-oxide impurities such as halogenswhich evaporate to a significant extent and which do not significantlydeteriorate the properties of the hot-pressed silicon nitride body mayalso possibly be present in amounts up to about 1% by weight of thestarting silicon nitride powder.

To produce a hot-pressed product which has substantially stablemechanical properties at high temperatures, the preferred startingsilicon nitride powder has a low oxygen content, i.e. usually about 2%or less by weight of the powder, and essentially free of elementalsilicon. Also, it is free or substantially free of metallic impuritiesin total amount ranging up to about 0.05% by weight of the powder. Sucha powder can be synthesized. Alternatively, to reduce its oxygen contentand also remove its vaporizable impurities, the silicon nitride powdercan be calcined at a temperature ranging from about 1300° C. to about1500° C. in a vacuum or in an atmosphere which has no significantdeteriorating affect on the powder such as helium, nitrogen, hydrogenand mixtures thereof.

The present silicon nitride powder can be produced by a number ofprocesses. For example, in one process SiO₂ is reduced with carbon innitrogen below 1400° C. Still other processes react a silicon halidewith ammonia or a nitrogen and hydrogen mixture to obtain either Si₃ N₄directly or via precursors such as Si(NH)₂ which are converted to Si₃ N₄by calcination yielding silicon nitride which usually contains oxygenand halogens at a 1% to 3% by weight level. The powder can also besynthesized in a plasma from silicon vapor and nitrogen.

Very pure silicon nitride powder can be formed by a process set forth inSer. No. 756,241 filed of even date herewith in the names of SvanteProchazka and Charles D. Greskovich and assigned to the assignee hereofand which by reference is incorporated herein. Specifically, thiscopending application discloses reacting silane and an excess amount ofammonia above 500° C. and calcining the resulting solid at between 1100°C. to 1500° C. to obtain amorphous or crystalline silicon nitride.

In the present process magnesium silicide, Mg₂ Si, is used as adensifying agent. In contrast to magnesium nitride which is highlyhygroscopic at room temperature which requires it to be used undernitrogen glove box conditions, magnesium silicide is a friable solidwhich is stable in air at room temperature and therefore presents noproblems with respect to formulation and mixing procedures therebypermitting a significantly simpler and much more practical preparationprocess. In the present process magnesium silicide is used in an amountranging from about 0.5% by weight to about 3.0% by weight of the siliconnitride. The preferred amount of magnesium silicide is determinableempirically and it is the lowest amount necessary to produce thehot-pressed body of desired density under the particular hot-pressingconditions used in the present process. However, amounts of magnesiumsilicide less than about 0.5% are not effective in producing the presenthot-pressed body with a density with at least about 80%. On the otherhand, amounts of magnesium silicide higher than about 3.0% by weight ofthe silicon nitride provide no additional densification of thehot-pressed body.

In carrying out the present process at least a significantly orsubstantially uniform or homogeneous particulate dispersion or mixturehaving an average particle size which is submicron of silicon nitrideand the magnesium silicide is formed. Such a dispersion is necessary toproduce a hot-pressed product with significantly uniform properties andhaving a density of at least 80%. The silicon nitride and magnesiumsilicide powders, themselves, may be of a particle size which breaksdown to the desired size in forming the dispersion but preferably thestarting silicon nitride is submicron and the starting magnesiumsilicide is less than 5 microns in particle size, and preferably,submicron. Generally, the silicon nitride powder ranges in mean surfacearea from about 2 square meters per gram to about 50 square meters pergram which is equivalent to about 0.94 micron to 0.04 micron,respectively. Preferably, the silicon nitride powder ranges in meansurface area from about 5 square meters per gram to about 25 squaremeters per gram which is equivalent to about 0.38 micron to about 0.08micron, respectively.

The silicon nitride and magnesium silicide powders can be admixed by anumber of techniques such as, for example, ball milling or vibratorymilling, to produce a homogeneous dispersion. The more uniform thedispersion, the more uniform are the microstructure and properties ofthe resulting dense hot-pressed body.

Representative of these mixing techniques is ball milling, preferablywith balls of a material such as tungsten carbide or silicon nitridewhich has low wear and which has no significant detrimental effect onthe properties desired in the final product. If desired, such millingcan also be used to reduce particle size, and to distribute anyimpurties which may be present substantially uniformly throughout thepowder. Preferably, milling is carried out in a liquid mixing mediumwhich is inert to the ingredients. Typical liquid mixing mediums includehydrocarbons such as benzene and chlorinated hydrocarbons. Milling timevaries widely and depends largely on the amount and particle size of thepowder and type of milling equipment. In general, milling time rangesfrom about 1 hour to about 100 hours. The resulting wet milled materialcan be dried by a number of conventional techniques to remove the liquidmedium. Preferably, it is dried in a vacuum oven maintained just abovethe boiling point of the liquid mixing medium.

The present powder dispersion is hot-pressed in an atmosphere ofnitrogen which can range from atmospheric pressure to superatmosphericpressure, generally, up to about 5 atmospheres. The nitrogen inhibits orprevents significant thermal decomposition of the silicon nitride andthereby promotes its densification. In the present invention nosignificant weight loss due to the thermal decomposition of siliconnitride occurs. Gases such as argon or helium are not useful at thelower pressing temperatures, i.e. below about 1750° C. because they aretoo expensive for commercial use, and at temperatures close to or above1750° C. they would not prevent thermal decomposition of siliconnitride.

Thermal decomposition of silicon nitride may possibly occur during thehot-pressing cycle to leave elemental silicon in the product. By asignificant thermal decomposition of the silicon nitride herein it ismeant a decomposition which produces elemental silicon in thehot-pressed product in an amount higher than about 2% by volume of theproduct. This can be monitored by microstructural observation ofpolished sections of the hot-pressed body.

The nitrogen gas used should be free of oxygen or substantially free ofoxygen so that there is no significant oxygen pickup by the body beinghot-pressed.

In carrying out the present process, the particulate mixture ordispersion is hot-pressed, i.e. densified, at a pressure and temperatureand for a sufficient period of time to produce the present denseproduct. Specifically, the hot-pressing temperature ranges from about1600° C. to about 1850° C. and applied pressure at such pressingtemperature ranges from about 2000 psi to a maximum pressure which islimited by available pressing equipment. Thus, for solid graphite diesthe upper limit is about 5000 psi and for graphite fiber-wound dies theupper limit is about 15,000 psi. The specific temperature and pressureused is determinable empirically and depends largely on the powder beingpressed and the specific dense product desired. The higher the pressure,the lower is the pressing temperature required, but as a practicalmatter, temperatures below 1600° C. will not produce the present denseproduct. On the other hand, temperatures higher than about 1850° C. arenot practical since the silicon nitride decomposes substantially in thepresent hot-pressing process at about 1900° C. resulting in materialloss. Preferably, for best results, the hot-pressing or densificationtemperature ranges from about 1700° C. to about 1830° C. and thepressure ranges from about 5000 psi to about 10,000 psi. It isadvantageous to use a pressure close to the maximum available becausethe application of such high pressure makes it possible to keep thepressing temperature low enough to control grain growth. Generally,hot-pressing in the present process is carried out at the desiredtemperature in a period of time ranging up to about 30 minutes, andlonger periods of time usually do not provide any significant advantageexcept at temperatures below 1700° C. where there continues to be aconversion of α- to the preferred β-form of silicon nitride.

The composition of the silicon nitride in the present product depends onthe hot-pressing temperatures used and ranges from α-silicon nitridealone to β-silicon nitride alone with all mixtures of the α- and β-formsof silicon nitride falling within the range. Specifically, withhot-pressing temperatures below about 1680° C. the silicon nitride inthe resulting hot-pressed product may be all of the α-form, or it may becomprised of a major amount of the α-form and up to about 25% by weightof the β-form, based on the total amount of silicon nitride depending onthe amount of α- which converts to the β-form, and also on the amount ofthe β-form initially present in the powder. At temperatures above about1680° C. and ranging up to about 1750° C., the silicon nitride in theresulting product is always a mixture of α- and β-forms of siliconnitride. At hot-pressing temperatures above about 1750° C., the siliconnitride in the product is usually only of the β-type.

The morphology of the α- and β-silicon nitrides in the hot-pressedproduct is distinguishable. Specifically, as determined by scanningelectron microscopy and metallographically in combination with X-raydiffraction analysis, the grains of α-silicon nitride are substantiallyequiaxed in form whereas the β-silicon nitride grains are elongated inform. The α-grains are always less than 2 microns in size and normallyless than 1 micron in size. Preferably, they have a grain size of 1micron or less. The β-grains are generally less than about 5 microns inlength, and for best results usually less than about 2 microns in lengthand have a width usually less than about 0.5 micron. The strength of thepresent hot-pressed product increases with increasing content ofβ-silicon nitride provided that such grains are less than about 10microns in length. The β-grains are interpenetrating usually forming anetwork which resists fracture. At relatively high hot-pressingtemperatures and for relatively long periods of hot-pressing, i.e.longer than about one hour, the β-grains may grow to a length of about10 microns. Preferably, the present product is comprised of only theβ-form of silicon nitride since it provides the most stable properties.

The hot-pressed body of the present invention has a density ranging fromabout 80%, and preferably from about 96% to about 100% of thetheoretical density of silicon nitride. The product is comprised ofsilicon nitride and some form of magnesium. The magnesium is present inan amount ranging from about 0.3% by weight to about 1.9% by weight ofthe silicon nitride. The magnesium component of the product isdetectable or determinable by techniques such as X-ray fluorescentanalysis, emission spectroscopy and chemical analysis.

The present hot-pressed product may also contain oxygen in some form inan amount up to about 3% by weight of the product. Preferably, for hightemperature applications, the hot-pressed product contains oxygen in anamount less than about 2% by weight of the product. Oxygen content maybe determined by techniques such as neutron activation analysis.

According to X-ray diffraction analysis or optical microscopy, thehot-pressed product may be single phase or polyphase. With reference tothe hot-pressed product of the present invention by the term singlephase or primary phase it is meant herein the silicon nitride phase,i.e. the α-form or β-form of silicon nitride and mixtures thereof. Thesingle phase hot-pressed body indicates dissolution of the magnesiumtherein. Generally, when the magnesium silicide additive is used inamounts of up to about 1% by weight of the silicon nitride, it isdifficult to detect a secondary phase in the hot-pressed product and itis believed to be a single phase material. However, when the magnesiumsilicide is used in amounts of about 2% to 3% by weight of the siliconnitride a secondary magnesium containing phase may be detected, and itis believed to be some form of magnesium silicon nitride. This secondaryphase may be present in an amount less than about 5% by volume of thehot-pressed body.

In an alternative embodiment of the present invention, free carbon,submicron in size, is admixed with the present silicon nitride andmagnesium silicide dispersion to form at least a significantly orsubstantially homogeneous dispersion. Mixing can be carried out by thesame techniques used in forming the silicon nitride and magnesiumsilicide dispersion. The amount of free or elemental carbon ranges fromabout 0.5% by weight to about 4% by weight, and preferably from about 1%by weight to about 2% by weight, based on the amount of silicon nitride.The particular amount of free carbon used is determinable empiricallybut should be sufficient so as to leave no free carbon or no significantamount of free carbon in the hot-pressed product. The free carbonaddition provides a number of advantages. It reacts with surface oxidefilms which may be present on the silicon nitride powder and also withoxygen present in the hot-pressing environment thereby substantiallyreducing the amount of oxygen in the final hot-pressed product. The freecarbon also reacts in the system to produce in situ very fine-sized puresilicon carbide particles less than about 2 microns in size. Thepresence of these fine silicon carbide particles in the silicon nitridematrix leads to improved strength and fracture toughness because thethermal expansion coefficient of silicon carbide is larger than that ofsilicon nitride and hence the silicon nitride matrix surrounding theparticles will be in a state of compression, thereby enhancing thefracture energy as well as the high temperature strength of theresulting hot-pressed product. In this embodiment of the invention, thehot-pressed product contains discrete and isolated particles of siliconcarbide distributed substantially uniformly throughout the hot-pressedbody in an amount ranging from about 1% to about 8% by volume of thebody. Amounts of free carbon which result in the formation of siliconcarbide in amounts higher than about 8% by volume of the hot-pressedbody are not useful because the larger volume of silicon carbideparticles would result in these particles being insufficiently isolatedto provide the desired state of compression.

In addition, if the starting powder contains free silicon, thehot-pressed body may also contain free silicon as a secondary phase, butsuch free silicon should be present in an amount less than about 2% byvolume of the hot-pressed body.

The secondary phase or phases are discrete and distributed substantiallyuniformly throughout the present hot-pressed body. Generally, the grainsof the secondary phase or phases are of about the same size or finerthan the grains of the primary phase.

The presence of a glassy phase is usually determined by selectiveetching of the specimen and observing the pits formed by the the etchedout glassy phase and/or by deep etching of the grain boundariesthemselves. Sectioning and polishing of the present hot-pressed body andsubjecting the polished surface to acid solutions containinghydrofluoric acid reveals no etching or no significant etching of thegrain boundaries which signifies essentially no detectable evidence ofan intergranular silicate phase at the grain boundaries.

The present hot-pressed product usually exhibits a preferred orientationof the grains in a direction perpendicular to the direction of theapplied hot-pressing pressure, i.e., a preferred orientation in theplane perpendicular to the hot-pressing direction. As a result, a testbar cut perpendicular to the hot-pressing direction usually exhibits atensile strength higher than that of a test bar cut parallel to theplane of the hot-pressing direction.

The present hot-pressed product is usful in structural applications suchas components for gas turbines. Specifically, the present process canproduce hot-pressed products in the form of simple shapes includingcylinders, plates and domes which retain their room temperature shapeand mechanical properties at high temperatures making these bodiesparticularly useful for high temperature structural applications.

In the present invention, unless otherwise stated, the density of thehot-pressed body is given as a fractional density of the theoreticaldensity of silicon nitride (3.18 g/cc).

The invention is further illustrated by the following examples whereinthe procedure was as follows unless otherwise stated:

In-house silicon nitride powder was prepared for use in all the examplesas disclosed in copending Ser. No. 756,241 filed of even date herewith.Specifically, this powder was prepared in a furnace which included anopen-ended fused silica reaction tube 3.8 cm. diameter placed in a tubefurnace, i.e. except for its open-end portions the reaction tube waslocated inside the furnace, and connected on the downstream end to acoaxial electrostatic separator operated between 5 and 15 KV and 0.2 to0.5 mA. The outlet of the separator was terminated with a bubbler filledwith an organic solvent which ensured positive pressure in the system. Aliquid manometer indicated gas pressure in the reaction tube. For eachrun the reaction tube was heated at a length of 15 inches to a maximumtemperature which was 600° C. for the silicon nitride powder used inExample 1 and which was 850° C. for the powder used in Examples 2 and 3,the system purged with purified argon and the reactants were thenmetered in. Electronic grade silane and anhydrous ammonia dried furtherby passing the gas through a column of calcium nitride were metered inseparately by coaxial inlets into the reaction tube. The gas flow rateswere adjusted to 0.2 standard cubic feet per hour (SCFPH) of SiH₄ and3.5 SCFPH of NH₃. A voluminous, light-tan powder was collected in thedownstream end of the reaction tube and in the attached electrostaticseparator. After 4 hours the gas flow of reactants was discontinued andthe system was left to cool off to room temperature under a flow of 0.5SCFPH of purified argon, and the powder was then recovered from thereactor and separator. The product was a light-tan powder, amorphous toX-rays, had wide absorption bands in its I.R. spectra centered around10.5 and 21.0 microns (characteristic for silicon-nitrogen bonding), andcontained no metals above 50 ppm determined by emission spectroscopy.

Surface area measurements were made by a low temperature nitrogenabsorption technique.

Oxygen content was determined by neutron activation analysis.

Powder density was determined by a helium Null-Pycnometer.

Before each hot-pressing run the system was evacuated and back-filledwith nitrogen gas and during the hot-pressing run and subsequent furnacecooling nitrogen was flowing through the system at a flow rate of 1cubic foot per hour.

EXAMPLE 1

The in-house silicon nitride powder used in this example had a specificsurface area of 15m² /g, a powder density of 2.75g/cc and an oxygencontent of 3.12% by weight of the starting powder.

To 1 gram of the silicon nitride powder there was added 0.03g Mg₂ Si,i.e. 3 weight % of magnesium silicide powder which corresponds to 1.9weight % of elemental magnesium based on the amount of silicon nitride,10 cc of benzene with 0.02 gram of paraffin added as a binder. Mixingwas carried out at room temperature for 15 minutes in a silicon carbidemortar and pestle in air.

After drying in a vacuum oven at 50° C. and collection, resulting drypowder dispersion, which had an average particle size which wassubmicron, was loaded into a graphite die fitted with a 1 cm. diameterboron nitride insert. The faces of the graphite plungers were coatedwith a boride nitride slurry and dried before hot-pressing. The boronnitride material prevented reaction between the silicon nitride andgraphite.

The thermal and pressure cycle for hot-pressing consisted of applying apressure of 3.5 MPa(500 psi) at room temperature and a pressure of 55MPa (˜8000 psi) at 1100° C. There was a 1 minute hold at red heat (˜800°C.) to remove the paraffin binder. The time to reach 1750° C. was about15 minutes. After a soak time of 20 minutes at 1750° C. in the nitrogenatmosphere under a pressure of 8000 psi, the power to the inductioncoils was turned-off, and the load removed. The boron nitride wasremoved by grinding it off the resulting hot-pressed body beforecharacterization.

X-ray diffraction analysis of the hot-pressed body showed that thesilicon nitride phase was composed of about 50% by weight β-siliconnitride phase and about 50% by weight α-silicon nitride phase; no otherphases could be detected. However, observation of a polished section ofthe hot-pressed body by optical microscopy at high (500-1000×)magnifications showed the presence of unidentified discrete second phaseparticles usually less than 10 microns in size and in total amount lessthan 2% by volume of the body. One of the secondary phases had a highreflectivity and was probably elemental silicon; the other secondaryphase is believed to be MgSiN₂. The residual pores were less than 5microns in size and usually smaller than 2 microns. The β-siliconnitride grains were elongated and not longer than 5 microns, with anaverage grain size of about 2 microns and an average aspect ratio lessthan 4. The density of the hot-pressed body measured to be 3.18g/cc, or100% of the theoretical value of 3.18g/cc. This run, i.e. the 3 wt.% Mg₂Si run, which corresponds to 1.9 wt.% elemental magnesium, is shown inthe accompanying figure.

In the accompanying figure all of the plotted runs were carried out inthe same manner as the 3 wt.% Mg₂ Si run except for the amount of Mg₂ Siused. Specifically, the control run at 0% Mg₂ Si produced a product witha density of 57%, at 0.5 wt.% Mg₂ Si (0.3 wt.% elemental Mg) the productdensity was 74%, at 1 wt.% Mg₂ Si (0.6 wt.% elemental Mg) the productdensity was 87%, and at 2.0 wt.% Mg₂ Si (1.3 wt. % elemental Mg) theproduct density was 100%.

The hot-pressed product produced in those runs where the magnesiumsilicide ranged from 0.5 wt.% to 2.0 wt.%, showed substantially the samesilicon nitride grain structure as was seen for the 3 wt.% Mg₂ Si run.However, the product produced in the 0.5 wt.% Mg₂ Si was found to besingle phase by X-ray diffraction analysis and optical microscopy.

The shape of the curve in the accompanying figure will be in partdetermined by the mixing procedure used in that better mixing procedureswill in general give higher densities for a given composition especiallyfor magnesium silicide levels less than 1 weight % because it isdifficult to disperse small amounts of the magnesium silicide with amortar and pestle.

EXAMPLE 2

The in-house silicon nitride powder used in this example was amorphousto X-rays and had a specific surface area of 16m² /g. This powder wascalcined in N₂ at 1450° C. for 15 minutes to essentially crystallize allof the powder into α-Si₃ N₄. The oxygen content and specific surfacearea of this calcined powder was 2.06 wt.% and 10.0m² /g, respectively.

40 g of this calcined, α-Si₃ N₄ powder was mixed at room temperaturewith 0.80 g of Mg₂ Si, i.e. 2 wt.% Mg₂ Si powder, 25 cc of a solution of1% paraffin in benzene, and 250 cc of benzene in a polyethylene jar millcontaining 1/4 inch balls of Si₃ N₄ grinding media. After mixing for 2hours, the resulting dispersion was dried in a vacuum oven at 50° C. for1 day. The powder mixture, which was a significantly homogeneousdispersion with an average particle size that was submicron, wascollected and placed in a 2 inch diameter graphite die which waspreviously coated with a boron nitride slurry and dried. The faces ofthe die plungers were also coated with a boron nitride slurry and dried.

The powder mixture was hot-pressed at 1800° C. for 15 minutes in anitrogen atmosphere at 10,000 psi. The density of the resultinghot-pressed body was 3.18g/cc, or ˜100% of the theoretical value. Thishot-pressed body was cut into a number of bars and evaluated.

X-ray diffraction analysis showed the sample was composed of β-Si₃ N₄plus a trace of α-Si₃ N₄.

Observation of a polished section by optical microscopy showed themicrostructure contained a small amount of secondary phases of sizesmaller than 5 microns. Particles of a bright reflecting second phasewere observed and believed to be elemental silicon, micron to submicronin size, and present in an amount less than about 1% by volume of thehot-pressed product. Particles of another secondary phase could be seenwhich is believed to be MgSiN₂ and present in an amount of less thanabout 2% by volume of the body.

The hot-pressed product had a Knoop hardness equal to 2250 kilograms persquare millimeter for a load of 200 grams.

Its resistance to oxidation was measured in air at 1410° C. for 21 hoursand the resulting weight gain was 2.0 milligrams per cm².

Observation of the grains in the microstructure by chemical etching (8minutes in boiling NH₄ F + HNO₃ mixture) reveals that the β-Si₃ N₄grains are elongated and have an average length of about 2 microns andan aspect ratio of about 4. It was difficult to etch most of the grainboundaries indicating no obvious detection of a glassy phase.

The fracture strength of the hot-pressed product in the 3 point bendingmode was determined. Its room temperature strength had an average valueof 64,000 psi in air and at 1350° C. in air it had an average value of52,000 psi indicating a retention of about 80% of its room temperaturestrength value.

This indicates that at 1350° C. the hot-pressed body of the presentinvention should maintain at least about 75% of its room temperaturestrength in air at 1350° C. in air. In contrast, commercially availablehot-pressed silicon nitride containing magnesium oxide as adensification aid usually retains only about 40% of its room temperaturestrength at 1350° C. in air.

EXAMPLE 3

The calcined silicon nitride powder disclosed in Example 2 was also usedin this example.

40 g of this calcined, α-Si₃ N₄ powder was mixed with 1.2g of Mg₂ Sipowder and 0.8 g of free carbon of submicron size, 25 cc of a solutionof 1% paraffin in benzene, and 250 cc of benzene in a polyethylene jarmill containing 1/4 inch balls of Si₃ N₄ grinding media. This wasequivalent to 3 wt.% Mg₂ Si and 2 wt.% free carbon based on the weightof silicon nitride. After mixing for 2 hours, the resulting dispersionwas dried in a vacuum oven at 50° C. for 1 day. The powder mixture,which was a significantly homogeneous dispersion with an averageparticle size that was submicron, was collected and placed in a 2 inchdiameter graphite die which was previously coated with a boron nitrideslurry and dried. The faces of the die plungers were also pre-coatedwith a boron nitride slurry and dried.

The powder mixture was hot-pressed at 1800° C. for 20 minutes in anitrogen atmosphere at 10,000 psi. The density of the hot-pressed samplewas (3.19 g/cc) which is slightly higher than the theoretical value(3.18g/cc) of pure Si₃ N₄, probably indicating the formation of SiC inthe hot-pressed body.

X-ray diffraction analysis showed the sample was composed of β-Si₃ N₄,plus a trace of α-Si₃ N₄. There was no detection of free C, SiC or anyother possible secondary phases.

Observation of a polished section by optical microscopy showed themicrostructure contained a small amount of secondary phases of a sizesmaller than 5 microns. Specifically, observation of the polishedsection by optical microscopy under reflected light shows the siliconnitride phase, i.e. the primary phase, as dark grey, and in contrastthere is a micron to submicron dispersion of a discrete lighter greyphase which is believed to be silicon carbide and which appears to bepresent in an amount less than about 4% by volume of the hot-pressedproduct. In addition, there appeared to be a few bright reflectingdiscrete particles of a phase which is believed to be elemental siliconand it appears to be present in a trace amount. In an occasional siliconcarbide grain there is evidence of a small unreacted particle ofelemental carbon and it is believed that such carbon can be eliminatedwith more uniform mixing.

Observation of the grains in the microstructure by chemical etching (8minutes in boiling NH₄ F + HNO₃ mixture) revealed that the β-Si₃ N₄grains are elongated and have an average length less than 2 microns.

All of the following cited applications are, by reference, made part ofthe disclosure of the present application and are assigned to theassignee hereof:

In copending U.S. patent application Ser. No. 756,084 entitled "HotPressing of Silicon Nitride Using Beryllium Additive", filed of evendate herewith in the names of Charles D. Greskovich, Svante Prochazkaand Chester R. O'Clair, there is disclosed a dense polycrystallinesilicon nitride body produced by hot-pressing a particulate mixture ofsilicon nitride and beryllium additive.

In copending U.S. patent application Ser. No. 756,085 entitled"Sintering Of Silicon Nitride Using Be Additive", filed of even dateherewith in the names of Svante Prochazka, Charles D. Greskovich,Richard J. Charles and Robert A. Giddings, the disclosed processcomprises forming a particulate dispersion of silicon nitride andberyllium additive into a green body and sintering it at a temperatureranging from about 1900° C. to about 2200° C. in a sintering atmosphereof nitrogen at super-atmospheric pressure producing a sintered body witha density ranging from about 80% to about 100%.

In copending U.S. patent application Ser. No. 756,086 entitled"Sintering Of Silicon Nitride Using Mg And Be Additives", filed of evendate herewith in the names of Svante Prochazka, Charles D. Greskovich,Richard J. Charles and Robert A. Giddings, the disclosed processcomprises forming a particulate dispersion of silicon nitride, magnesiumadditive and beryllium additive into a green body and sintering thegreen body at a temperature ranging from about 1800° C. to about 2200°C. in a sintering atmosphere of nitrogen at superatmospheric pressureproducing a sintered body with a density ranging from about 80% to about100%.

What is claimed is:
 1. A hot-pressed polycrystalline silicon nitridebody having a density ranging from about 80% to about 100% of thetheoretical density of silicon nitride, said body consisting essentiallyof silicon nitride, magnesium and oxygen, said silicon nitride rangingfrom the α-form to the β-form with all mixtures of said α-and β-formsfalling within said range, said magnesium being present in an amountranging from about 0.3% by weight to about 1.9% by weight of saidsilicon nitride, said oxygen being present in an amount ranging up toabout 3% by weight of said body, said polycrystalline body ranging froma single phase body to one comprised of a primary phase and less thanabout 5% by volume of said body of a secondary magnesium-containingphase, said body being at least substantially free of an intergranularsilicate glassy phase at its grain boundaries and retaining at leastabout 75% of its room temperature mechanical properties at elevatedtemperatures ranging up to about 1350° C. in air.
 2. A polycrystallinesilicon nitride body according to claim 1 having a density ranging fromabout 96% to about 100% of the density of silicon nitride.
 3. Apolycrystalline silicon nitride body according to claim 1 wherein saidsilicon nitride is present in the β-form.
 4. A hot-pressedpolycrystalline silicon nitride body having a density ranging from about80% to about 100% of the theoretical density of silicon nitride, saidbody consisting essentially of silicon nitride, magnesium, oxygen andsilicon carbide, said silicon nitride ranging from the α-form to theβ-form with all mixtures of said α-and β-form falling within said range,said magnesium being present in an amount ranging from about 0.3% byweight to about 1.9% by weight of said silicon nitride, said oxygenbeing present in an amount ranging up to about 3% by weight of saidbody, said polycrystalline body being composed of a primary phase, up toless than about 5% by volume of said body of a secondarymagnesium-containing phase, and from about 1% to about 8% by volume ofsaid body of particles of silicon carbide less than about 2 microns insize distributed substantially uniformly throughout said body.